Display tmux window ID in message if available

[Azenet]

This is a simple pull request for displaying the window ID in the message when a process ends and contains the appropriate environment variable.

[franciscolourenco]

@Azenet can you share an example of what this looks like in practice? Thanks!

[Azenet]

@franciscolourenco It adds [@2] at the beginning of the notification if the process was running in window 2, for example.

[Azenet]

I was using the wrong variable for the display, which became apparent after using tmux for a long time in which windows were closed (id stays unique). Also added a configuration variable for the format, defaults to [2] for window 2 for example.

[Azenet]

Sure, here is what the notification looks like after sleep 10 has ended in pane 30:

Rebased and made the requested change.

[franciscolourenco]

Thank you @Azenet! This will be released in v1.17.0.